PROJECT SUMMARY The primary visual cortex has the most accurate map of visual space in the brain and devotes about a square millimeter of its surface to represent all stimulus parameters needed to process a single point in an image. The list of stimulus parameters represented for each point is extensive and includes spatial position, contrast polarity (light or dark), orientation, direction of motion, binocular disparity, eye input, and spatial frequency. Research over the past decades has revealed with increasingly more detail the cortical representation of different stimulus parameters, however, the precision of the current visual cortical maps is still closer to a 150 AD Ptolemy-map of the world than a Google map. We still have a limited understanding of the precise arrangement of different stimulus representations and relations between representations within the map and we still do not know why animals with high visual acuity have cortical maps arranged in intriguing topographic patterns (e.g. orientation pinwheels) that are not existent in animals with poorer vision. Our work over the past years started to reveal the principles underlying visual cortical topography and their possible functional implications. Based on this work, we recently proposed that the topography for all stimulus parameters originate from the arrangement of four different types of thalamic afferents in visual cortex, which convey information about light-dark polarity (ON and OFF) and eye input (contralateral and ipsilateral). The orderly arrangement of these afferents by spatial position, eye input and contrast polarity provides a smooth and accurate map of visual space but at the expense of representing some stimulus parameters more accurately than others (e.g. darks better than lights) and compromising the representation of multiple stimulus parameters when the visual field is large. This proposal takes advantage of novel multielectrode-arrays to map with unprecedented precision the topography of primary visual cortex representing a point of visual space. We combine multielectrode recordings from thalamus and cortex with pharmacological methods and computational modeling to dissect the thalamic and cortical networks underlying cortical topography and the relative contribution of ON and OFF pathways to the structure and function of visual cortical maps.